JPS5914925B2 - FM demodulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention proposes an FM demodulator with a new configuration.

FIG. 1 shows the basic configuration of the present invention, and in FIG.
For example, 8 is f.

This shows a modulated wave signal source that is FM modulated with .

A resistor R8 and a coil L are connected in series to this signal source e8.

A series resonant circuit consisting of a capacitor C8 and a capacitor C8 is connected,
Lead out the terminal t1 from the connection point of the resistor R8 and the series resonant circuit, and coil it.

A terminal t2 is derived from the connection point of the capacitor C6.

The resonant frequency of this series resonant circuit is f.

has been selected. Also, coil.

A coil L1 and a capacitor C1 are connected in parallel with a series resonant circuit including a capacitor C6 and a series resonant circuit.

The first signal voltage e1 obtained at the terminal t1 with the above configuration
The absolute value 1e11 of is as shown in FIG. 2A.

That is, the frequency f of the modulated wave is the resonant frequency f of the series resonant circuit.

When it is equal to , the impedance of this series resonant circuit becomes zero, so 1e11 becomes zero.

And the frequency f is f. If it becomes lower (when the actance component of the series resonant circuit becomes capacitive, the capacitor C1 is added to this and the coil L1 resonates in parallel, and the impedance becomes infinite at the resonant frequency f1, and 1e11 becomes the input voltage e8 is equal to

If the frequency f becomes further lower, 101
1 becomes zero.

On the other hand, the frequency f is f.

When the voltage becomes higher, the actance component of the series resonant circuit becomes inductive, and the sum of the coil L1 and the capacitor C1 resonate in parallel, and at the resonant frequency f2, 1e11 becomes equal to the input voltage e8, and the frequency further increases. As f becomes higher, 1e11 becomes zero due to capacitor C1.

Further, the absolute value 1e21 of the second signal voltage e2 is as shown in FIG. 2A due to the capacitor C3jC, the coil Ll j LO and the resistor Ro.

The maximum value of 1e21 is determined by the Q of the resonant circuit, and the larger the resistor R8, the smaller the Q becomes.

Here, the signal voltage e2 is expressed as follows using el.

Then, finding the values of el and C2, we get Therefore, the resonant frequency f.

At lower frequencies, the output e is obtained by introducing the first signal voltage e1 and the second signal voltage e2 into a well-known multiplier, multiplying them, and passing them through a low-pass filter.

As shown in FIG. 2B, exhibits a figure-8 characteristic with respect to the frequency f of the modulated wave, and the modulated wave e8 can be FM demodulated.

The FM demodulator according to the present invention described above has a frequency f at which the demodulated output is zero.

However, coil L. It is determined only by the capacitor co and is not affected by the coil L1, capacitor C1 multiplier, etc., and is extremely stable in terms of direct current.

Also, the frequency f. In the vicinity of , components that are integral multiples of the carrier wave do not appear in the multiplication output, so there are few unnecessary signals and the load on the low-pass filter is light.

Further, the present invention has the advantage that a demodulation transformer is not used and the coil configuration is simple.

In this respect, the quadrature FM demodulator has similar effects, but the other effects of the present invention described above cannot be expected from the quadrature demodulator circuit.

Hereinafter, one embodiment of the present invention will be described with reference to FIG.

In FIG. 3, Ql and C2 indicate transistors forming a differential amplifier, and a DC bias is applied to their bases.

The collector of transistor Q1 is connected to the power supply terminal (+vcc) via a load resistor, and the above-mentioned coil L
1 and one end of the parallel circuit of the coil capacitor C1 and the coil L.

and connected to one end of the series resonant circuit of C8.

Terminals t1 and t2 are led out from both ends of the series resonant circuit,
The other end is connected to the emitter of transistor Q3.

The emitter of transistor Q3 is grounded via an emitter resistor, its base is connected to DC power supply E1, and its collector is connected to the base of transistor Q2.

Furthermore, an input terminal t3 for receiving the fl wave is connected to the emitter of the transistor Q3 via a capacitor.

Coil in the positive feedback loop of the differential amplifier consisting of the above-mentioned transistor QI C2.

A capacitor CO is inserted to form an oscillator.

This oscillator oscillates under the condition (ωgLoCo=1).

In this state, when a modulated wave with an angular frequency ω and an amplitude A is supplied from the terminal t3, the amplitude A is large to some extent and ω
But ω.

(-2πfo), the current of the differential amplifier is constant, so the amplitude is constant regardless of the amplitude A, and ω
It oscillates at an angular frequency of

Then, the output of terminals t1 and t2 is the transistor Q4
~Q9 is supplied to the multiplier.

The multiplier has a well-known double-balanced configuration, and a multiplication output is obtained at its output terminal t4.

By passing this multiplication output through a low-pass filter, a demodulated output is obtained as described above.

In one embodiment of the present invention described above, by configuring an oscillator and inserting a series resonant circuit into its oscillation loop, the AM component of the modulated wave can be suppressed without going through a limiter.

Furthermore, even if the modulated wave is interrupted, it oscillates at an angular frequency of ω0, so the demodulated output is ω.

It operates in the same way as if a modulated wave were supplied, and the demodulated output becomes zero.

This is advantageous when demodulating a SECAM carrier color signal.

In other words, in the S E CAM system, the carrier color signal is FM modulated and sent line-sequentially, but there is no carrier color signal component during the synchronization signal period, and in many cases there is noise due to switching. do.

If such a signal is FM demodulated by suppressing the AM component with a limiter as in the past, the signal component will be suppressed, resulting in poor S/N in the synchronization signal section, but in the present invention, the carrier wave C0 is used even in this section. has occurred, so S/ in this section
N can be made good.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a basic connection diagram of the present invention, FIG. 2 is a route diagram used for explaining the same, and FIG. 3 is a connection diagram of an embodiment of the present invention. Lo and C8 are a coil and a capacitor forming a series resonant circuit, el and C2 are first and second signal voltages, C8
is the modulated wave signal source.

Claims (1)

[Claims] 1 in series with the modulated wave signal source via a resistor.
A series resonant circuit consisting of an inductance element and a first capacitance element is connected, and a parallel resonant circuit consisting of a second inductance element and a second capacitance element is connected in parallel to this series circuit. The resonant frequency is selected as the center frequency of the modulated signal, and for frequencies higher than the center frequency, the total parallel resonant frequency of the series resonant circuit and the parallel resonant circuit is higher than the center frequency, and the center frequency is set to the center frequency. For lower frequencies, the values of the second inductance element and the second capacitance element are selected so that the overall parallel resonance frequency is lower than the center frequency, and the series resonance with the resistance is selected. A first signal voltage obtained at the connection point of the circuit and a second signal voltage obtained at the connection point of the first inductance element and the first capacitance element.
An FM demodulator, characterized in that the signal voltage is supplied to a multiplier, and an FM demodulated output is obtained from the multiplier.